A formation tester is the oil well logging tool which is lowered on a cable into a well borehole to make hydrostatic as well as formation pressure measurements and obtain formation fluid samples. This normally includes a sonde and the appropriate instrumentation which is enclosed within the sonde. All of this equipment is lowered into a well borehole for making fluid pressure measurements at various depths in the well. Ordinarily, such measurements are made under a standing column of fluid in the well. The fluid is a mixture typically derived from drilling mud, and further including salt water and oil which might be produced from various formations into the well. Drilling mud itself is water or oil based which has been mixed with various weight materials, one example being barites. Whatever the source, the column of fluids standing in the well is ordinarily a dirty fluid. It is important to make accurate pressure measurements and to this end, the preferred form of device for such measurements is a quartz crystal pressure transducer. Quartz transducers that measure pressure utilize a quartz wafer as the resonantor in an oscillator circuit. Pressure exerted along the circumference of the resonator changes the resonant frequency of the circuit. The circuit including the quartz crystal operates at relatively low voltages. To this end, the crystal must be provided with carefully constructed voltage contacts or terminals on it, and they are normally formed with gold leaf which is deposited on the crystal. Accordingly, the crystal itself and the gold lead clad to the surface of the crystal wafer define a delicate instrument. They are delicate, and they are especially so in view of the risk which arises from placing this device in the dirty fluids in the well borehole.
The crystal is located in a cavity within the sonde which is filled with an electrically non-conductive fluid. This fluid filled cavity is connected to the flow line of the formation test tool through a "J" shaped buffer arrangement which provides the location (housing or container) for the interface between the fluid contained within the cavity and the well bore fluid flowing into the flow line of the formation tester. Ideally, the fluid contacting the crystal placed in the crystal chamber is non-conductive, and does not cause corrosion of the crystal, i.e., it is non-corrosive.
Corrosion often occurs when the well fluid contacts the crystal or the gold leaf clad terminals placed on the crystal. The present apparatus sets forth a mechanism which prevents such contact. Moreover, the present apparatus has a special feature which holds down on shock loading. For reasons which may arise in any formation testing operation, a pressure shock wave may travel in the fluid contained within the flow line. The shock wave traveling through the incompressible fluid in the flow line may damage the crystal. The present apparatus includes along and circuitous flow path which is between the well fluid in the flow line and the crystal. This long and circuitous flow path dampens any shock waves. Moreover, the crystal is protected from exposure by this circuitous pathway, meaning exposure to the shock waves and also to the impinging trash and other materials which are laden in the fluids in the well borehole.
The present apparatus is a device which is relatively easy to assemble. It is formed of threaded members, there being three members which are concentrically arranged. This defines a long helical flow path which has a purge valve inlet at one location and an open end at the other. Moreover, it is shaped in the form of a "J" flow path. Heavy particulate trash and the like will settle at the bottom area of the "J" and not flow any further.
The present invention, summarized briefly above, has added protection against the intrusion of trash, corrosive fluids, electrically conductive fluids, and other things which might otherwise damage this expensive measuring device. More will be noted regarding this structure on a review of the below written specification which is considered in conjunction with the drawings attached hereto and in conjunction with the claims set forth thereafter.